Method of protectively coating uranium



METHOD OF PROTECTIVELY COATING URANIUM Lowell D. Eubank, South Euclid,Ohio, and Ernest R. Boiler, Marion, Ind., assignors to the United Statesof America as represented by the United States Atomic Energy CommissionNo Drawing. Application March 14, 1946 Serial No. 654,504

Claims. (Cl. 117-51) This invention relates to the coating of metallicuranium. Coated metallic uranium articles are useful as fuel elements inneutronic reactors of the type described in the co-pending applicationof Fermi et al. Serial -No. 568,904, filed December 19, 1944, now PatentNo. 2,708,656, granted May 17, 1955.

Early attempts to coat metallic uranium with zinc met withunsatisfactory results because of the failure of the zinc to coat themetal uniformly. Coatings applied by dipping the metallic uraninum intomolten baths of zinc of the composition normally employed in theapplication of Zinc coatings to steel and similar metals resulted inonly spotty coverage of the metallic uranium. In accordance with theteachings of our United States patent application Serial Number 588,060,filed April 12, 1945, now Patent No. 2,848,796, granted August 26, 1958,we have found that satisfactorily adherent coatings of zinc can beapplied provided a substantial proportion of aluminum is present in themolten zinc so as to form a uranium-aluminum alloy layer between themetallic uranium and the zinc. For some purposes, the alloy coatings ofzinc containing aluminum are as satisfactory as zinc coatings orsuperior to zinc coatings. For some purposes, however, the presence ofaluminum is objectionable.

An object of the present invention is the provision of firmly adherentprotective coatings on metallic uranium. A further object is theprovision of such coatingssuitable for use as bases for the applicationof other metal coatings. A yet further object of the invention is theprovision of zinc coatings on metallic uranium without the use ofsubstantial proportions of aluminum in the zinc coating alloy to providea uranium-aluminum alloy interlayer. 1

In accordance with the present invention it has been discovered that byemploying a zinc which is free from lead and iron impurities, continuouscoatings of zinc on metallic uranium may be applied without the use ofan aluminum alloy. Commercial zinc normally employed for theapplication. of hot dip coatings may contain up to 1.6% of lead and upto 0.08% of iron. Such zinc compositions result in the spotty coatingspreviously referred to. We have found that by employing a zinc whichcontains less than 0.01% of lead and less than 0.01% of iron andpreferably less than 0.01% total of lead, iron and cadmium, very uniformcontinuous coatings of zinc firmly adhering directly to the uranium maybe applied.

The coatings of the invention provide very substantial protection formetallic uranium against the corrosive effects of the atmosphere andother active reagents.

For special purposes the zinc-coated uranium may be given a secondcoating with another metal more resistant to the corrosive influencesparticularly concerned. Thus coatings of lead containing a smallproportion of silver or tin, or aluminum containing a minor proportionof silicon may be applied over the zinc coatings.

The coatings may be applied by dipping the uranium to be coated into abath of zinc at a temperature between about 430 C. and 600 C. The bathmay be provided with a standard flux, preferably as alkali-metalchloride flux melting at substantially below bath temperature, or it maybe devoid of flux. 7

The uranium may be prepared for coating by sandblasting oracid-pickling, preferably in warm strong nitric acid.

The following examples will illustrate the invention.

Proportions are in terms of weight if not otherwise expressed.

Example 1 A metallic uranium rod about two inches long and three-eighthsof an inch in diameter was immersed in aqueous HNO solution at roomtemperature forabout 5 minutes to clean the metal surface and rinsed inwater. The rod was then dipped for 30 seconds into a bath of pure zinccontaining less than 0.007% lead, less than 0.005 iron, less than 0.005%cadmium, and more than 99.99% zinc. The zinc bath was at a temperatureof 540 C. and was covered with a molten flux compris-' Example 2 Fivemetallic uranium rods were cleaned in 50% nitric acid solution asdescribed in Example 1 and dipped into molten zinc baths of the samecomposition as employed in Example 1 using the same flux. Thetemperatures of the molten metal baths were 555 0, 505 C., 450 C I 440C., and 433 C. In each case a uniform, continuous coating of the zincdirectly on the metallic uranium was obtained.

When an attempt was made to coat metallic uranium with an ordinary hotdip coating zinc (Prim-e Western Spelter) under the conditions describedin this example,- only spotty coatings could be obtained. Portions ofthe metallic uranium surface remained exposed after the coatingoperation in every case.

The following examples illustrate the application of I multiple coatingsemploying azinc coating as base.

Example 3 Example 4 A metallic uranium rod, after cleaning in aqueous50% nitric acid solution, was dipped for 30 seconds in a zinc bath ofthe composition employed in Example 1, maintained at 560 C. and coveredwith a 37% lithium chloride, 53% potassium chloride, 10% sodium chlorideflux. The rod was withdrawn from the bath, cooled in air for 10 seconds,and then rolled on Transite rolls until the coating solidified. Thecoated rod was quenched by immersion in water. The resulting rod, whichwas uniformly coated with zinc, was then dipped for 30 seconds into amolten lead bath comprising 98% lead and 2% PatenteclFeb. 3, 1959 Ametallic uranium rod prepared for coating as described in Example 1 wasdipped for 1% minutes in a zinc bath maintained at 500 to 510 C. Thezinc bath was of the same high purity as employed in Example 1.

After 1% minutes in this bath the rod was removed and centrifuged for 8seconds at 640 revolutions per minute in a centrifuge in which it wassuspended coaxially at a distance of about 6 inches from the axis ofrotation. The

rod was'then dipped into a bath comprising 88% aluminum and 12% siliconat a temperature of 620625 C. for between and 15 seconds, removed anddipped for 3 seconds into a second aluminum-silicon bath of the samecomposition maintained at 585 C. The rod was withdrawn from this secondbath, rolled for 15 seconds on Transite rolls and then quenched inwater. The rod was completely coated with aluminum-silicon alloy over athin zinc interlayer.

It will be understood that we intend to include variations andmodifications of the invention and that the preceding examples areillustrations only and in no wise to be construed as limitations uponthe invention, the scope of which is defined in the appended claims,wherein we claim:

1. The method of protectively coating uranium, which comprises preparingthe uranium for coating and dipping the freshly prepared metal into abath of molten zinc containing less than 0.01% each of iron and lead,said bath being at a temperature between 430 C. and 600 C., withdrawingthe uranium coated with zinc and cooling it to solidify the coating.

2. The method of protectively coating uranium, which comprises picklingthe uranium in strong nitric acid and dipping the freshly prepared metalinto a bath of molten zinc containing less than 0.01% each of iron andlead,

said bath being at a temperature between 430 C. andsaid bath beingv at atemperature between 430 C. and 600 C., withdrawing the uranium coatedwith zinc, dipping the zinc-coated uranium into a molten bath comprising88% aluminum and 12% silicon at a temperature of about 620-625 C. forjust sufficient time to provide complete coating, withdrawing the coateduranium from the aluminum-silicon bath and cooling it to solidify thecomposite coating.

4. The method of protectively coating uranium, which comprises picklingthe uranium in strong nitric acid and dipping the freshly prepared metalinto a bath of molten zinc containing less than 0.01% each of iron andlead, said bath being at a temperature between 430 C. and

600 C., withdrawing the uranium coated With zinc, dipping thezinc-coated uranium into a molten bath comprising approximately 90% leadand 10% tin at a temperature of approximately 350 C. withdrawing thecoated uranium and cooling it to solidify the composite coating.

5. The method of protectively coating uranium, which comprises picklingthe uranium in strong nitric acid and dipping the freshly prepared metalinto a bath of molten zinc containing less than 0.01% each of iron andlead, said bath being at a temperature between 430 C. and 600 C.,withdrawing the uranium coated with zinc, dipping the zinc-coateduranium into a molten bath comprising approximately 98% lead and 2%silver, said bath being ata temperature below the melting point of zinc,withdrawing the coated uranium and cooling it to solidify the compositecoating.

References Cited in the file of this patent UNITED STATES PATENTS547,381 McKnight Oct. 1, 1895 927,372 Monnot July 6, 1909 1,114,792Monnot Oct. 27, 1914 1,497,265 Haas June 10, 1924 1,597,189 Gero Aug.24, 1926 1,869,784 Theiss Aug. 2, 1932 1,982,563 Wimmer Nov. 27, 19342,231,967 Tainton Feb. 18, 1941 2,276,101 Schueler Mar. 10, 19422,299,166 Miller Oct. 20, 1942 2,307,243 Slagle Jan. 5, 1943 2,396,730Whitfield et a1 Mar. 19, 1946 OTHER REFERENCES Metals Handbook,- 1939edition, page 1736. Protective Coatings for Metals, Burns and Schuh,1939, pages 60, 61, 64, 65, 94 and 101.

1. THE METHOD OF PROTECTIVELY COATING URANIUM, WHICH COMPRISES PREPARINGTHE URANIUM FOR COATING AND DIPPING THE FRESHLY PREPARED METAL INTO ABATH OF MOLTEN ZINC CONTAINING LESS THAN 0.01% EACH OF IRON AND LEAD,SAID BATH BEING AT A TEMPERATURE BETWEEN 430*C. AND 600*C., WITHDRAWINGTHE URANIUM COATED WITH ZINC AND COOLING IT TO SOLIDIFY THE COATING.